1. Field of the Invention
The present invention relates to an image-forming device, and particularly to a control of the power supplied to transferring means provided in the image-forming device.
2. Description of Related Art
In an image-forming device, electrical resistance in the transferring means, image-carrying member, and recording paper interposed between the transferring means and image-carrying member can fluctuate greatly due to ambient conditions (particularly temperature and humidity). Hence, the image-forming device must adjust the power supply to adapt to changes in the ambient conditions. An insufficient power supply may cause toner to scatter on the recording paper due to an insufficient force for depositing the toner. Further, toner that is not transferred onto the recording paper but remains on the image-carrying member may be mistakenly transferred onto the recording paper at another position. Conversely, if the power supply is excessive, electrical discharge may result in damage to the image-carrying member.
A method of controlling power supply has been disclosed in Japanese unexamined patent application publication No. HEI-6-308844. In this method, a characteristic curve for the optimal voltage and measured current is prepared for each output voltage applied to the transferring means. The characteristic curve corresponding to the current output voltage is selected, and control is performed based on this characteristic curve in order to achieve an optimal output voltage corresponding to the currently measured current value.
However, Japanese unexamined patent application publication No. HEI-6-308844 does not explicitly describe how the characteristic curve is derived, and it has been difficult to determine whether optimal power control can be achieved.